Objective-C block的參數(shù)問題: This block declaration not a prototype

我在學(xué) ReactiveObjC, ReactiveCocoa的Objective-C版.
RACSignal.h 中，以下代碼:,

- (RACSignal *)reduceEach:(id _Nullable (^)())reduceBlock
RAC_WARN_UNUSED_RESULT; (id _Nullable (^)())

Xcode 報錯:

This block declaration is not a prototype

reduceBlock()能放很多參數(shù).
UIAlertView+RACSignalSupport.m 中，以下代碼， 其他文件也有 ,

- (RACSignal *)rac_buttonClickedSignal {

RACSignal *signal = [[[[self.rac_delegateProxy
    signalForSelector:@selector(alertView:clickedButtonAtIndex:)]
    reduceEach:^(UIAlertView *alertView, NSNumber *buttonIndex){
        return buttonIndex;
    }]
......
return signal; }

有點(diǎn)泛型的感覺。
void (^block)() 這樣聲明，就能在block 放0到多個參數(shù)了.
Xcode現(xiàn)在不支持這種語法，怎樣解決，解釋下更好。